The invention relates to a method and apparatus for focusing and imaging x-rays. More particularly, the invention relates to extremely high resolution imagery such as that required for high density micro-lithography.
Methods of focusing x-rays and imaging x-ray sources or objects illuminated with x-rays are quite different from focusing and imaging visible light. Because the refractive index of materials is very close to unity at x-ray wavelengths, refractive lenses are not possible as with visible light.
Curved mirrors, used at grazing incidence to the x-rays, are currently used to focus x-rays; however these mirrors are costly to fabricate because of their non-spherical topography. These mirrors are also difficult to align. By depositing multi-layer structures to curved mirrors they can be made to operate at normal incidence, but to achieve high resolution imaging the surfaces must be extremely smooth, on the order of the wavelength of the x-rays. In addition, these mirrors are made for a specific wavelength of light and imaging configurations, limiting the uses of these mirrors. Also, mirror imaging systems have a short depth of field. This means that the image is in focus at a discrete location, and if the image plane is slightly moved from the location where the image is in focus, the image would be out of focus.
Fresnel zone plates are used to image x-rays by diffraction. A Fresnel zone plate looks like a very small bull's eye pattern with alternating opaque and transmissive rings. The separation of the outer most rings determines the imaging resolution, the resolution being approximately equal to 1.22 times the separation of the outer most rings. Zone plates are fabricated using photolithography or micro-machining techniques. The inability of these techniques to construct Fresnel zone plates to a desired tolerance severely limits the ultimate resolution that can be obtained. To provide an image with a resolution of 700 angstroms, a zone plate would need the outer most rings to be spaced approximately 575 angstroms apart. In addition, zone plates have a short depth of field. Zone plates are also limited with respect to imaging objects that are off axis with respect to the zone plate. The further away an object is from the axis the more out of focus the image for that object becomes.
The diffraction of visible light by an opaque sphere was first demonstrated by Arago in 1818. The distribution of light in an observation plane was derived by Mie in 1908 using rigorous electromagnetic theory. The resultant formulas, however, are quite involved and cannot easily be used to study the characteristics of diffraction pattern. Some authors have tried to described the pattern using scalar diffraction theory. Most have concentrated on the axial intensity of the central bright spot, with Osterberg and Smith solving the Rayleigh diffraction integral exactly. In 1914 Hufford realized that this diffraction phenomenon could also be used to form an image. He demonstrated this using a carbon arc lamp, but never pursued the problem analytically nor with non-visible radiation. Since then, this phenomenon has received little attention, since for visible imaging applications other high quality lenses and mirrors are readily available.